1. Field of This Invention
The invention relates to a process for the production of a mixed-oxide oxidation catalyst, which, on the basis of vanadium and pentavalent phosphorus, has an atomic ratio of phosphorus to vanadium between 1.05 to 1 and 1.10 to 1, and to a process for the use of the mixed-oxide obtained according to the process as a catalyst for the production of maleic anhydride from C.sub.4 -hydrocarbons of a saturated or unsaturated kind.
2. Prior Art
The production of vanadium-phosphorus mixed oxides and their use as oxidation catalysts is known. German Published Application No. 2,256,909 describes the production of a vanadium-phosphorus mixed oxide catalyst, having an atomic ratio of phosphorus to vanadium of 1:1 to 2:1, by the complete concentration of a solution of a pentavalent phosphorus compound and a vanadium compound in concentrated aqueous hydrochloric acid and subsequent formation with the help of a complicated multistep heat treatment. The exact control of the heat treatment process with regard to temperature, duration in time and atmosphere of the surroundings is given as the prerequisite for achieving sufficient catalytic activity of the resulting dehydrated mixed oxide. In the case of the technical and commercial production of catalysts, it is however exceedingly difficult and expensive to maintain (keep within) such complicated conditions as the process of such German published application.
German Published Application No. 2,328,755 describes the production of an oxidation catalyst having a high specific surface and a special crystallic structure, which is designated as .beta.-phase and is defined by its x-ray difraction spectrum. At the same time the preliminary catalyst product, that is to say the uncalcined catalyst, is obtained by heating and completely concentrating a practically anhydrous hydrochloric acid solution of an approximately tetravalent vanadium compound and a 100 percent phosphoric acid. Water can only be present in small quantities, whenever the active mixed oxide described by the German published application is to be obtained. For such formation, again a complicated multistep heat treatment, analogous to the previously mentioned German published application, is necessary. Beside the already discussed disadvantage of the demanding heat treatment, the necessity of the use of organic solvents is disadvantageous.
The C.sub.4 -hydrocarbons, especially n-butane, owing to their considerably lower price, are economically superior to benzene which at the present day is customarily used for the large-scale industrial production of maleic anhydride.
In the case of the mixed-oxide catalysts obtained according to the two above-mentioned German publications, quite high yields are achieved in oxidation of n-butane to maleic anhydride. The specific through-puts of n-butane used in such case, however, are far below those customarily obtained in industry and required for a reasonably profitable commercial process.
The additional known processes of the prior art either use only very small turnovers and return the unreacted starting material into the process, which is conditional on the supply (feeding in) of pure oxygen to the reaction mixture (for example, German Published Application No. 2,354,872), or else they require temperatures of above 500.degree. C. in order to achieve mere moderate yields, a factor which considerably reduces the useful life of the catalyst.